Resin-encapsulated devices currently predominate in the semiconductor industry. Epoxy resins are generally superior to other thermosetting resins in terms of moldability, adhesion, electrical characteristics, mechanical characteristics, and moisture resistance. Epoxy resin compositions are commonly used for the encapsulation of semiconductor devices.
Ball grid array (BGA) packages, developed fairly recently by Motorola, have a distinctive structure in which the chip is mounted directly onto the circuit board substrate, and the top of the chip is encapsulated in plastic. In BGA packages, only one side of the substrate is resin encapsulated. Hence, the difference in shrinkage factor between the substrate and the resin leads to warping of the package, which is a major problem.
A number of attempts have been made to overcome this problem by increasing the glass transition temperature (Tg) and lowering the thermal expansion coefficient of the resin so as to reduce the difference in shrinkage between the substrate and the resin, and thus minimize package warp. One specific solution involves using a polyfunctional epoxy resin, a polyfunctional phenolic resin as the curing agent, and an imidazole compound as the curing accelerator in order to increase the glass transition temperature, and including also a large quantity of silica filler to lower thermal expansion. In order to enable high loading of silica filler while maintaining good flow characteristics, it is known to use all spherical silica particles free of fragments so as to optimize the particle size distribution of the filler. A method of treating silica with a coupling agent to optimize its surface state is also known. Another important property when evaluating device reliability is the adhesion of the resin to the solder mask covering the substrate surface. It is well known in the art that adhesion of the resin to the solder mask can be dramatically enhanced by the judicious selection of an epoxysilane or mercaptosilane coupling agent.
Yet, the prior art described above was found to have a number of serious drawbacks. For instance, the use of an epoxy resin and a phenolic resin both of the polyfunctional type results in a cured product having an increased water absorption because of an increased free volume within the molecular structure. As a result, the cured product becomes low in soldering heat resistance after moisture absorption and susceptible to popcorn cracks. Also, all non-crystalline epoxy resins including polyfunctional ones have a relatively high viscosity. When an epoxy resin composition is loaded with a large quantity of an inorganic filler such as silica, the composition has an increased melt viscosity which when a BGA package is encapsulated therewith, causes molding defects such as wire flow and breakage. Additionally, epoxy resin compositions using an imidazole compound as the curing accelerator have a shelf stability inferior to that of compositions using a phosphorus-containing accelerator, so that wire flow and incomplete filling due to a rapid rise in viscosity in the resin encapsulation step are more likely to arise unless the resin encapsulating step is strictly managed. In addition, hydrolyzable chlorine within the epoxy resin is more readily extracted, which can be detrimental to device reliability in the presence of moisture. Furthermore, the steady increase in package size within the industry requires that further reductions be made in thermal expansion, but the high loadings of silica currently in use increase the viscosity of the composition, resulting in frequent wire flow. A certain type of coupling agent slows the curing speed of the epoxy resin composition at the time of resin encapsulation, which can lead to an increase in package warp.
Effective solutions have not previously been found to these and other problems associated with prior-art resin compositions for BGA encapsulation.